Wednesday, April 15, 2009

N4JTE 6 BAND 'RIBBON" ANTENNA $35

N4JTE; 6 BAND “ RIBBON” $35 ANTENNA------------------------------------------------------------------------------------------This antenna article is geared towards new Hams and antenna builderslooking for a very inexpensive 6 band antenna that can be efficiently fedwith 50 ohm coax without a tuner.The inspiration for this design resulted from a visit to my 82 year oldneighbor’s home who had asked me for some help in dismantling hisamazing and increditably beautiful model train set, and box up for hisgrandson. During this process I was intrigued by his use of 5 and 10conductor 18 gauge flat insulated ribbon cable for all of the L.V. switchingand action devices.So Begins the Adventure.

To be honest I am getting more and more frustrated with some of the latestmarketing ploys being used by commercial antenna manufacturers and theirincredible, misleading and unsubstantiated miracle “all band” antennas thatwill sucker in some poor unsuspecting new ham who will spend his moneyon a heavily marketed, overpriced, and in some cases, Amazingly reviewedantenna // Toaster.Mt plan was to use this relatively cheap insulated wire and find out if wasactually possible to get 6 bands cleanly matched to 50 ohm coax. As thisantenna was basically built for testing and performance evaluation theconstruction details are limited and somewhat primitive by most standardsso I will leave it to others to refine and permatize, hi.When I envisioned this concept my only real concern was how all the closespaced wires would inter react. The shorter dipoles will all present highimpedance at the feed point when they are not driven forcing the feed line topick the path of least resistance and best match for the frequency. The 40meter wire will serve well on the 15 meter band as a center fed 1.5 wl wire.I am aware that a fan dipole uses the same single feed concept but I believethe Ribbon antenna eliminates all those extra tie off points while maintainingthe resultant extra effective radiated height, especially if used in a flat topconfiguration. Certainly less obtrusive and much less work.BUILDING IT: Well you have to start somewhere, so I chose the OMISS net frequenciesavailable at www.omiss.net, as my starting point for the band frequenciesand wire lengths. I have built more than enough monoband Inv. Vees at thislocation with insulated wire so I use my own formula of 450/ freq. toachieve what I’m after with minimal pruning.You need to approach this antenna one side at a time. The ribbon wire Ipurchased was only available in 50 ft. lengths so I knew the 75 meter wirewould need about 8 more feet to each end to reach 75 meters. It’s best tounroll and stretch the wire out to remove the “wire memory”. Thataccomplished measure out your 5 chosen ¼ wl lengths and mark or tape off.As the 75 wire is pretty much done after adding the required wire, separatethe next wire, and carefully peal back to the taped off marker and cut offexcess. Continue this process for the remaining wires up to the 10 meterpoint. Yeah I know it seems like a lot of wasted wire but at 6 cents a footyou’ll get over it !Repeat the process with the other half of your antenna. I just laid them alongside each other and matched all the lengths. Be careful to use a dull knife orfingernail file to separate the wires so as to not break the insulation.After cutting all your wires to length you will need to have some kind ofcenter support and feedline connector in mind before stripping and solderingthe 5 conductors together. In my case I pushed each one thru a ceramicinsulator and then carefully stripped and soldered together in preparation forfeedline attachment.The ends of half wave dipoles are at high rf voltage and if too close to otherswill add unwanted capacitance and tuning problems. For starters I justseparated the adjacent wires by about a foot and let them dangle down.FIRST ATTEMPT.My goal with this initial attempt was to see basically where or if ,I would getfull power out. This would give me the best indication of what was actuallyradiating and at what frequency. You cannot expect, nor limit yourself, to a1 to 1 swr as being your goal. The meter will serve only as a guideline towhere your wire length and height works in the real world of your backyard.A quick look at any antenna book will show the relationship between heightabove ground and radiation resistance in ohms. This inv. Vee at 43 ft. iseven a little more tricky to predict especially when the 75 meter antenna isonly at .175 wl high and the 10 meter antenna is at 1.2 wl above real ground.All 6 antennas should range between 20 and 90 ohms with the ends at 10 ft.above ground. The results of the first attempt were very interesting in that atleast I was getting full power out somewhere close to the 6 bands of interest.I had my doubts because of introducing the 17 meter antenna into the mixwhich is not an even multiple of the lowest band, usually considered a no noon multiband antennas.--------------------------------------------------------------------------------------------SECOND ATTEMPT:I use an eye hook stuck into the top of the fiberglass pole with a masonrystring to allow easy up and down access for the cutting and tuning process. Istarted by adding about 4 ft to each end of the 75 wire and after somediddling ended up with around 3.95m. the only band affected by this changewhen scanning thru the bands was the 40 meter wire which changed it’sapparent resonant frequency to 6.9M and the 15 meter wire also dropped infrequency. A quick trip to the backyard to shorten each end of 40 wire byone foot made no change on the resonance. I then separated the forty fromthe 75 by dangling about 3 ft. at each end. Obviously the end effect waskicking in because I ended up at 7.1m, close enough for my needs and the15 meter wire was happy in the middle of the SSB portion.FIRST TRY RESULTS75----------4.179m short40----------7.290m short20----------14.190m okay17----------17.800m long15----------21.553m long10----------27.713m longSECOND TRY.75--------------3.95m40--------------7.1m20--------------14.190m17--------------17.800m15.--------------21.300m10--------------27.717mTHIRD ATTEMPT:The only bands left to fine tune were the 17 and 10 meter wires which werestill too long, so I cut off 6 ins. From both ends of each antenna. The 17meter ended up at 18.135 and the 10 meter at 29.5 in the FM portion of theband which I actually prefer these days..RECAP:The whole tune/recut exercise took about 6 hours and resulted in a 6 bandantenna that will radiate full power out without a tuner. Due to the fact thatthere are no traps, no loading coils, no tuners and no ladderline needing abalun to match, the only losses will be in the feedline due to it’s length andnot the result of any mismatch at the antenna feedpoint. I do not have thenecessary brain power to model this design and would appreciate a peerreview on the modeled radiation resistance and resultant antenna patterns.Point of interest; The 60,30 and 12 meter bands had around a 3 to 1 swr andwere showing 60 to 70 watts out without a tuner. Probably work well withauto tuner.IF YOU BUILD IT:1; A simple Plexiglas T, or equivalent with double slots for the ribbon wireand two small holes to tie wrap the coax should be more than enough afterwaterproofing to ensure stability and strength for a center supported lightweight antenna such as this.2; The 75 wire ends up holding the whole antenna up, so I would attachsome masonry string to the center insulator and tie wrap at the band enddangles and a couple of more at the 40 and 80 wires with the string going totie off points. By tie wrapping you will also prevent the wires fromseparating at band junctions.3; Attach small non conductive weights at the drop wires after final tuning.THIRD, FINAL TRY, I’m lazy:75----------------3.95m40.---------------7.1m20----------------14.190m17----------------18.135m15----------------21.360m10----------------29.500m4; Use a tuner at higher powers to attenuate harmonics and any possiblespurious transmitter outputs.5; If 10 conductor wire is available you can double up the wires for eachband providing an increase in bandwidth and power handling. FYI, I had noproblems with the 5 wire at 500 watts ssb.FINAL COMMENTS:I am not going to waste everyone’s time by recounting all my log entrieswhile testing this antenna. I will tell you it is a joy to run from 75 to 10meter fm and be able to hear what is going on and respond to a cq withoutfumbling around with antenna switches and tuners. This antenna is nothingmore than 6 inv vees at 43 ft. that perform to the laws of physics and willserve you well if you are committed to a little sweat equity to get it workingefficiently.You will not be disappointed with this $35 antenna.BUILD ; DON’T BUY !Tnx for reading,Bob N4JTE

1 Comments:

KC2UQA here! I enjoyed the QSO tonight with you, Kurt and Gary. I just wanted to let you know I found your article on the multi-band ribbon antenna several weeks ago when I was researching antennas for my shack (I found it using a google search for multi-band antennas). I was intrigued then, and I am intrigued now. It is a very cool concept. Perhaps when I am ready to move towards a resonant antenna system I will build one. For now I am keeping it simple with the non-resonant approach. And btw, I was able to tune-up the 135 ft dipole on 40m to 1.9:1 tonight. Looks like I need to spend some more time reading the manual. And experimenting...